1. Field of the Invention
The present invention relates to a loop antenna and electronic equipment including the loop antenna. More particularly, the present invention is concerned with a loop antenna to be adapted to a device in or from which data is written or read in a non-contact manner, such as an IC card, and electronic equipment including the loop antenna, such as a portable cellular phone.
2. Description of the Related Art
In recent years, IC cards, having a built-in IC in which information such as data of receipts and payments of money can be stored, have prevailed. Information on a commuter pass, a role of an IC card, and information on stations at which a user of the IC card gets on or off are stored in the IC card. The user of the IC card may use the IC card as a computer pass or a ticket to pass through an automatic ticket gate in a railway station in a non-contact manner. Otherwise, the user of the IC card stores a credited (charged) sum in the IC card, and uses the IC card as digital cash to pay small sums of money at shops or convenience stores. Moreover, depending on information stored in the IC card, the IC card may be used as a staff ID card or a student ID card.
Generally, a cell that supplies power is not incorporated in the non-contact type IC card, but an IC chip on which a CPU and a memory are mounted, and a resonant circuit composed of a loop antenna and a resonant capacitor are incorporated therein. Weak electromagnetic waves sent from an external reader writer are caught by the loop antenna incorporated in the IC card, whereby power is generated and used to operate the IC chip. When reply data produced by the IC chip is radiated from the loop antenna, the reader writer recognizes the IC card. Mutual authentication is duly performed. If the result of authentication signifies that the IC card is authentic, data is read or written in or from the reader writer or the IC card. A series of reading/writing actions is performed instantaneously.
The IC chip and loop antenna have come to be incorporated not only in the IC card but also in electronic equipment, for example, a portable cellular phone. This reflects the trend of the reader writer being installed at kiosks or included in vending machines. When the IC chip and loop antenna are incorporated in the portable cellular phone, the portable cellular phone can be used as digital cash. This promotes the use of digital cash.
FIG. 6 included in Japanese Unexamined Patent Application Publication No. 2000-269725 shows the internal circuitry of an IC card. The IC card internally includes a loop antenna having taps to an IC chip at both ends of the loop antenna. Moreover, the loop antenna is divided into three portions due to the intermediate taps. A first capacitor is connected to the first portion of the loop antenna, whereby a first resonant circuit is formed. The second portion of the loop antenna has no connection. A second capacitor is connected to the third portion of the loop antenna, whereby a second resonant circuit is formed. The resonant frequencies of the first and second resonant circuits can be differentiated from each other by varying the capacitances of the included capacitors.
The thus configured IC card can be applied to various usages in addition to the aforesaid one. Therefore, a loop antenna is expected to have the communicative performance thereof improved, and to cope with the adverse effect of metal in the case where the loop antenna is incorporated in electronic equipment. Specifically, when the aforesaid loop antenna and IC chip are incorporated in portable electronic equipment such as a portable cellular phone, the loop antenna is susceptible to metal and, for example, the metallic part of a cell incorporated in the equipment. In this case, the cell alone has been shielded with a magnetic substance in the past.
Furthermore, a communication range within which the IC card can communicate with a reader writer varies depending on the reader writer employed. Moreover, the reader writer may be available in various types but not in all types. The IC card is therefore required to exhibit a stable performance in whichever of diverse usages the IC card is applied. Moreover, the loop antenna and IC chip are required to exhibit stable performance even if they are incorporated in electronic equipment, such as a portable cellular phone, other than an IC card.
As mentioned above, the IC card or electronic equipment in which the loop antenna and IC chip are incorporated is useful and promising but has the drawbacks described below.
(1) When only one loop antenna is incorporated together with an IC chip in a communication system, the detectable range offered by the communication system is limited.
(2) As a communication method utilizes electromagnetic coupling, when a loop antenna and an IC chip are incorporated in electronic equipment, the loop antenna is susceptible to metal included in the casing.
(3) As a resonant antenna unit is disposed in a narrow space in electronic equipment, a communication range is limited to an area defined by the space. Once the antenna unit is affected by any metal or radiation noise, even if the resonant frequencies of first and second resonant circuits are differentiated from each other, communication cannot be maintained satisfactorily.